ON THE CHANGE OF REFEANGIBILITY OF LIGHT. 361
false dispersion is afforded by chloride of tin dissolved in a very large quantity of common water.
182. When a horizontal beam of falsely dispersed light is viewed from above, in a vertical direction, and analysed, it is found to consist chiefly of light polarized in the plane of reflexion. It has often struck me, while engaged in these observations, that when the beam had a continuous appearance, the polarization was more nearly perfect than when it was sparkling, so as to force on the mind the conviction that it arose merely from motes. Indeed, in the former case, the polarization has often appeared perfect, or all but perfect. It is possible that this may in some measure have been due to the circumstance, that when a given quantity of light is diminished in a given ratio, the illumination is perceived with more difficulty when the light is uniformly diffused than when it is spread over the same space, but collected into specks. Be this as it may, there was at least no tendency observed towards polarization in a plane perpendicular to the plane of reflexion, when the suspended particles became finer, and therefore the beam more nearly continuous.
183. Now this result appears to me to have no remote bearing on the question of the direction of the vibrations in polarized light. So long as the suspended particles are large compared with the waves of light, reflexion takes place as it would from a portion of the surface of a large solid immersed in the fluid, and no conclusion can be drawn either way. But if tbe diameters of the particles be small compared with the length of a wave of light, it seems plain that the vibrations in a reflected ray cannot be perpendicular to the vibrations in the incident ray*. Let us suppose for the present,
[* The way in which I at the time regarded the problem was as follows. ^Suppose polarised light to he passing through a medium which holds in suspension a vast number of excessively fine particles of some substance different from the modi urn itself, the dimensions of the particles being for simplicity supposed extremely small compared with the length of a wave. The ether in the medium will ho vibrating to and fro in a direction perpendicular to the direction of propagation, and either in or perpendicular to the plane of polarization. The inertia of the particles being presumably very great compared with that of a corresponding volume of the ether alone, the ponderable particles may be supposed to remain at rest, and they will therefore disturb the motion of the ether, and cause vibrations to spread out from them in the ether. Now the repose of the particles may be regarded as the resultant of two equal and opposite motions, one